Momentarily baffled, I realized that, despite appearances, the old frame was actually not square, in fact it was a parallelogram. I'd measured the height and width and assumed it was square. The previous (experienced) carpenter who'd built the doors I was replacing had clearly noticed this, and simply allowed for the misalignment in his design. He built perfectly square-appearing doors that mounted to the not-square frame. I had to go back and rework mine considerably for them to fit without looking ridiculous. They're still there and holding up well, but I also still think of this lesson on a regular basis in my day to day life now.
Here's the thing we - a flat full of nerdy tech students - never figured out.
The walls in two of the bedrooms (including mine) were perfectly plumb, all four walls straight. Bookshelves lined up nicely with the walls. The floor was flat and level.
But the room was 10cm narrower at the ceiling.
If you built the bookshelf in wood, it will be expanding, contracting and shifting over time with temperature and humidity variation throughout the day and season. And asymmetrically depending on the grain.
The straight right angles won't stay that way, and it's better to design such that they change in complementary ways, rather than remain perfect.
The walls aren't straight, either vertically or horizontally, and they're not even consistently wonky along any given axis.
So I installed uprights vertically, using transparent polycarbonate spacers of different depths at the attachment points[0]. I then installed shelves on the uprights and aligned them horizontally.
The variation is only +/- 6mm or so (for around 12mm variation across the 2.5m x 2.44m wall) but, if I hadn't done this, my shelves wouldn't be level, and wouldn't even be consistently non-level, so would have been awkward to install along the full length of the wall, would all be misaligned with eachother, and would have looked incredibly janky.
[0] In hindsight I wish I'd gone for these in different colours rather than just plain transparent, to make more of a feature of them. The walls are white so I think orange, blue, red, and yellow would have worked well.
This must be a well-known fact to all trades people who work on cupboards, tiling, door mounting, etc. But when you understand this, then you realize that everything is built to be forgiving of this reality.
E.g. prefabricated bedroom cupboards will always be fitted with fillers on each side and a kickboard for the bottom. This allows you to use feet/wedges underneath the cupboard to make it stand-up perfectly straight (which is not necessarily parallel with the floor and/or walls), but because of the fillers/kickboard being wide/tall enough and cut to fit the irregular/skew shape, you don't tend to notice.
Beading around wooden door frames is for the same reason, it hides the little gap that is invariably at points around it, either due to the hole in the wall being skew and/or slightly arched.
I definitely prefer that with software it can be “perfect” and easily changed later if you find it’s not.
Also learned that lath and plaster needs some special consideration when screwing/nailing things for securement, as the lath (wood strips) could split, causing a subsequent crack in the plaster. Basically for screws or bigger nails, it’s a good idea to drill a small hole first to lessen the pressure, or do a bigger hole and use a spring bolt anchor.
Hate hate hate hate hate
I can never look at staircases the same.
Soon you realize that an surprising amount of walls are just not straight or level.
I put up a notice board in my kitchen when I moved into a new place, and it looked squint even though my level said it was straight. I flipped the spirit level end-for-end, still straight - if the level was off and the workpiece was straight, then flipping it would make it read wrong.
Nope. Level was okay. Checked the wall, wall is plumb. Wait a sec - the wallpaper is not straight.
So the notice board went on lined up with the wallpaper, not reality.
The other one was fitting a six metre aluminium pole with a two metre aerial on top to a brand new multi-million pound building. The brackets and pole were absolutely straight and plumb. Got back down off the cherry picker, walked back across the yard to the van, pole looks really squint.
After much upping and downing and to-ing and fro-ing, it became clear that I needed to pack the aerial pole mounting to lean it over by a couple of degrees so it didn't look wrong!
The pole was straight, the multi-million pound brand new high end amazing building was distinctly on the piss with not one truly plumb vertical component anywhere.
It's not the codes, but the physics. The first two years after a building is build, it will change it's geometry until it settles. That happens because building has a significant weight and the earth under the building was unsettled, and now is under a pressure.
Not very noticeable in a light weight houses, but even small brick one-family house will do that.
Building codes account for that, but it's better and significantly cheaper to build that way then to build a totally rigid structure. Rigid is brittle.